A number of electrical devices, such as various types of lamps, motors, actuators and sensors, are mounted on an automobile to perform various control and indicating functions, and the number of such devices provided on an automobile is on the increase.
In automobiles in current use, these electrical devices are wired individually. As a result, the wire bundles (known as the wire harness) are complicated and increase in size with the increase in the number of electrical devices, thereby posing a great problem in the design of the automobile. Specifically, both the cost and weight of the wiring harness have increased with the increase in complexity of the automobile electrical system, while the space for wiring within the automobile has decreased. Further, with recent advances in car electronics, a number of electronic devices, including a microcomputer, are now mounted on the car, with the result that signals transmitted by the wire harness include not only simple on/off signals, but also include numerous data signals of more complicated form, thus presenting the hazard of false operation of equipment as a result of data inversion of the transmitted signals caused by noise.
As a means of solving these problems, it has been suggested to adopt the technique of multiplex signal transmission which is already widely used in the field of communications for use in automobiles to transmit the many signals between devices by means of a small number of wires. In such a signal transmission system, optical fiber cables which are light in weight and non-inductive are used as a transmission path. Such a system is disclosed in Japanese Patent Laid-Open No. 105490/80 and No. 136149/83.
A data transmission system in the form of a collective wiring system is disclosed in Japanese Application No. 106666/83 (corresponding to U.S. application Ser. No. 619,998, filed Jun. 12, 1984), in which an optical fiber cable is used as a signal transmission path, so that a central control unit (hereinafter referred to merely as a "CCU") is coupled with a plurality of terminal processing units (hereinafter referred to merely as "LCU") by means of an optical signal channel. The CCU is arranged preferably at an appropriate point near the dashboard of the automobile to control the whole system. A predetermined number of LCU's are arranged, on the other hand, at locations near groups of electical devices mounted in the automobile, such as operating switches, display units, meters, lamps and sensors of various types. A photo-electric conversion module for two-way conversion of an optical signal and an electrical signal is provided at the part where the CCU and each LCU is connected to the optical fiber cable. In this system, the CCU is provided with a microcomputer and has a data communication function based on the transmission of serial data. In correspondence with this, each LCU has one or a plurality of communication processing circuits which form communication interface modules (hereinafter referred to merely as "CIM"). The CCU selects an LCU and then the CIMs in the LCU sequentially exchange data with a CIM thus selected. By repeating this, multiplex transmission with all of the CIMs becomes possible through the optical fiber cable, thus simplifying the complicated bulky wirings in the automobile. In such a data transmission system as this, the computer of the CCU (hereinafter referred to as "the micom") has the functions to analyze and process the data received from each LCU, determine the LCU to which data is to be transmitted next, while at the same time preparing transmission data containing control information for the particular LCU and applying the same to the CIM in the CCU.
In an application of this data transmission system to the collective wiring system of an automobile or the like, the series of control operations which the micom is required to perform must be carried out at sufficiently high speed in order to guarantee smooth control of the automobile or the like. In the case where a lamp is to flicker in response to the operation of an operation switch, for instance, the control operation of the micom in the CCU, if not as speedy as required, would cause delayed lamp flickering. This delay may pose a serious problem depending on the nature of the control.
The collective wiring system in an automobile, on the other hand, generally involves a very great number of loads (terminals) for data exchange, sometimes as many as several hundred, often requiring direct connection of the load to the CCU, and therefore not only data transmission control by the CIM but also many other various processing operations are required of the micom of the CCU. In the conventional collective wiring systems for automobiles using a data transmission system, therefore, with an increase in the number of loads, the processing capacity of the micom of the CCU may become insufficient causing a delayed control operation, and thereby making it impossible to maintain a desired level of control response. Thus, it is essential to provide for the micom a most efficient operating procedure.
Some automotive devices have an input not corresponding to an output, but are not operable unless a plurality of inputs are turned on (hereinafter referred to as AND-related inputs), or are operable when any one of a plurality of inputs is turned on (hereinafter referred to as OR-related inputs), or have a plurality of loads actuated upon turning on of an input. A first example includes a rear window defroster or a fog lamp. The rear window defroster is not activated unless the rear defroster switch is turned on while the ignition key switch is also on. The fog lamp, on the other hand, is capable of being energized in many cases only when the lighting switch is on. A second example is that of OR-related inputs, which include the interior lamp, the parking lights and the hazard warning light. The lighting switch and the hazard warning switch are a third example.
A method of realizing these configurations in the collective wiring system mentioned above is either by hardware or by software. In the method using hardware, a plurality of inputs are required to be processed by AND or OR devices, resulting in lengthy wires extending throughout the automobile to and from such logic devices, which tends to substantially reduce the advantages of the use of the collective wiring sytem. It is, therefore, necessary to realize the logic relationship between these inputs and the loads by software. Further, it is desirable to establish correspondence easily between the number of input and output points for each model of an automobile so as to make the system easily compatible with electrical systems of different configuration.